The peanut plant is produced in many countries, and is recognized as one of the major oilseed crops and as a rich source of protein. Peanuts are grown worldwide in the tropic and temperate zones, and although they are grown elsewhere primarily for the seed oil, in the United States peanuts are used mainly for human foods such as peanut butter, roasted seeds, and confections. The United States is consistently a major exporter of peanuts for human consumption. The unique roasted flavor of peanuts is the basis for most marketing of export/import peanuts. Thus, improvement of the factors that indicate and/or affect food quality of peanuts is of considerable importance to the worldwide peanut processing and manufacturing community.
The performance characteristics of a plant oil, for either dietary or industrial purposes, are substantially determined by its fatty acid profile, that is, by the species of fatty acids present in the oil and the relative and absolute amounts of each species. The type and amount of unsaturation present in a plant oil has important consequences for both dietary and industrial applications. With regard to edible peanuts, their final quality is due principally to the chemical composition of the oil, protein, and carbohydrate fractions of the seed, with the oil component playing a major role. Other factors such as kernel size, blanching characteristics, roasted flavor, and shelf life also contribute to peanut quality.
Plant oils are subject to oxidative degradation, which can cause highly undesirable changes in color and odor, as well as detract from the lubricity and viscosity characteristics of the oil. Color and odor are of particular concern in food applications, where the autoxidation of oils, and the accompanying deterioration of flavor, is referred to as rancidity. The rate of oxidation is affected by several factors, including the presence of oxygen, exposure to light and heat, and the presence of native or added antioxidants and prooxidants in the oil. However, of most pertinence to the present invention, and perhaps generally, is the type and amount of unsaturation of the fatty acids in the oil.
By way of one example, peanut butter consists of a mixture of solid nut particles and liquid peanut oil. It is generally made by roasting and blanching raw peanut kernels, followed by grinding. The grinding operation breaks the cellular structure of the peanut kernels and liberates oil; the comminuted nut particles are suspended in this oil to form a product having a pasty and spreadable consistency.
An unavoidable side effect of the processing, and particularly the grinding, is the introduction of oxygen into the peanut butter. The presence of this oxygen promotes the development of rancidity; the oxygen oxidizes the peanut oil, causing the peanut butter to develop an undesirable taste and/or odor during prolonged storage, thereby reducing the useful shelf life of the peanut butter.
The fatty acids present in the peanut oil are not equally vulnerable to oxidation. Rather, the susceptibility of individual fatty acids to oxidation is dependent on their degree of unsaturation. Thus, the rate of oxidation of linolenic acid, which possesses three carbon-carbon double bonds, is 25-100 times that of oleic acid, which has only one double bond, and 2-10 times that of linoleic acid, which has two. Schultz, E. A. Day and R. O. Sinnhuber, Symposium on Food Lipids and Their Oxidation, AVI Publishing Co., Inc., London, England (1962). Linoleic and linolenic acids also have the most impact on flavor and odor because they readily form hydroperoxides. While saturated fatty acids are the most stable from the viewpoint of rancidity, they are highly undesirable from a health and nutrition viewpoint. Therefore, monounsaturated fatty acids present the best combination of nutritive and oxidative stability characteristics.
Resistance to oxidation can also have an important benefit before the peanut is crushed for oil or incorporated into a food product. There appears to be a correlation between susceptibility to oxidation and the formation of aflatoxin, an extremely undesirable contaminant which can result from the growth of certain molds. It has been reported that lipoperoxidation of the unsaturated fatty acids--particularly linolenic acid--in sunflower seeds appears to be involved in aflatoxin biosynthesis. Passi et al., "Role of lipoperoxidation in aflatoxin production," Applied Microbiology Biotechnology, pp. 186-190 (1984); Fanelli et al., `Free radical and aflatoxin biosynthesis," Experientia, 40:191-193 (1984). Therefore, peanuts whose oil contents are resistant to oxidation should also be resistant to aflatoxin formation.
The botanical classification of peanuts is subject to some apparent ambiguity, with sub-divisions being variously referred to as varieties, subvarieties, subspecies, classes, and types. Thus, commercial peanuts can be divided into at least two main botanical classes, Virginia and Spanish, and possibly a third, Valencia (Norden et al., "Breeding of the Cultivated Peanut," Peanut Science and Technology Ch. 4, pp. 95-121 (1982). Some sources use a different third main class, Runner (Peanuts: Production, Processing, Products, ch. 4, "The Culture of Peanuts," pp. 41 et seq., Woodroof, ed. (3d ed. 1983), while others appear to treat Runner as a sub-class of Virginia (Wynne et al., "Peanut Breeding," Advances in Agronomy, 34:39-72 (1981). (It appears that some of the apparent confusion may result from the use of overlapping nomenclature in taxonomic and commercial naming systems.) For purposes of the present discussion, Runner will be treated as a sub-class derived from the Virginia botanical class of peanut.
Virginia botanical types do not flower on the main stem and, in general terms, mature later, have a high water requirement, and are large-seeded. Spanish botanical types flower only on the main stem and, relative to Virginia types, mature earlier, have a lower water requirement, and have smaller seed.
Virginia peanuts are the more desirable for comestible applications, due to their generally superior size, texture, and taste compared to Spanish peanuts. In addition, oil from seeds of the different botanical types of peanuts differ in their tendency to develop oxidative rancidity and its associated undesirable odors and flavors. Virginia-type peanuts produce oil with a lower linoleic percentage, and therefore tend to have greater stability than Spanish types. Moreover, Virginia type peanut plants are superior to Spanish in several agronomic characteristics, including drought resistance, disease and pest resistance, and susceptibility to pod splitting.
In general, the genetic range of fatty acid compositions in peanuts is 41-67% oleic acid and 14-42% linoleic. O'Keefe et al., "Comparison of Oxidative Stability of High- and Normal-Oleic Peanut Oils," J. Am. Oil Chem. Soc., 70:489-492 (1993), referencing Ahmed et al., Peanut Science and Technology, pp. 655-688 (1982). The Runner sub-class of Virginia-type peanut is the basis for the most widely used commercial peanut varieties in the United States, due to its excellent agronomic characteristics. A recent survey of Runner-type peanuts reported an oleic acid content range of 49.6-56.3%, and a linoleic acid content range of 24.1-30.6%. Branch et el., J. Am. Oil Chem. Soc., 67:591 (1990).
One widely used commercial variety of the Runner sub-class is Florunner, which was introduced in the United States in 1969 as commercial runner type peanut derived from a cross between the varieties Early Runner and Florispan. Florunner constituted about 90% of all runner-type peanut grown in the U.S. in the 1970's, and still had about 70% of this market in 1992. Usage of Florunner in 1994 is estimated at between 60% and 70% of all runner-type peanut in the U.S., and 25 years after its introduction Florunner remains the most cultivated peanut trade variety in the United States, and is the standard against which every new introduction is measured. However, the oil of Florunner peanuts contains about 51% oleic fatty acid and 29% linoleic fatty acid, yielding an O/L ratio value of only about 1.76.
Prior attempts have been made to derive a high oleic/low linoleic variety having a Florunner background. Norden, French Patent Application No. 2,617,675, assigned to the University of Florida, describes a Florispan-derived peanut seed with an oil content of approximately 74-84% oleic acid and 2-8% linoleic acid. This application claims priority from U.S. Ser. No. 071,881, filed Jul. 10, 1987, which does not appear to have issued as a U.S. patent.! However, the derivation of this peanut seed apparently included an early cross between a Virginia-type female parent and a Spanish-type male parent. This would be expected to yield offspring plants having substantially less desirable agronomic characteristics than a runner variety. Moreover, the seeds of such offspring would unavoidably reflect at least some of the inferior size, taste, and texture of Spanish peanuts; it appears, in fact, that they were classified as commercial Spanish-type peanuts under the U.S. Marketing System.
In a 1987 journal article, "Variability in Oil Quality Among Peanut Genotypes in the Florida Breeding Program," Peanut Science. 14:7-11, Norden et al. describe the lengthy efforts made by the Florida breeding program to improve the oil quality of Arachis hypogaea L. This paper was apparently a forerunner to filing of the above-referenced U.S. patent application. It describes the same two lines which form the basis of that application, 435-2-1 and 435-2-2, as being derived from seed stock which was a Florispan derivative, and acknowledges the possibility of a subsequent Spanish outcross based on "variation in seed characteristics."
In a follow-up to the Norden et al. work by Knauft et al., "Further Studies On The Inheritance of Fatty Acid Composition in Peanut," Peanut Science. 20:74-76 (1993), the authors, working from the F435 line, conclude that the high oleic acid trait in peanut follows simple genetic inheritance, such that "incorporation of high oleic acid into peanut cultivars should be straightforward." However, there is no acknowledgment that F435 is a Spanish genotype, and no recognition or discussion of whether the authors' conclusion, if accurate, would apply to incorporation of the high oleic acid trait into non-Spanish backgrounds. Given the commercial dominance of runner-derived peanut lines, the current peanut production system, and the need to maintain the existing agronomic and flavor characteristics of the premium commercial varieties, introgression breeding of the high O/L trait from other varieties into Florunner is a difficult and uncertain approach which is unlikely to be commercially feasible.
It would, therefore, be particularly beneficial to develop a variety of Virginia-type peanut having the desirable agronomic characteristics of the Runner class, and in particular the Florunner variety, yet also having oil with a high content of monounsaturated acids and a low content of saturated and polyunsaturated fatty acids. The superior oxidative stability of oil from such a peanut variety would provide benefits in both industrial and comestible uses, where increased shelf life would decrease the incidence of rancidity and improve product quality, not only of the oil per se, but also of food products containing the peanuts in whole or reduced form.
Moreover, there would be very substantial practical benefits if a commercial high oleic peanut line was made available having agronomic characteristics substantially identical to those of Florunner. Because of the long-standing dominance of Florunner, every step in the chain of peanut production and marketing is highly calibrated to handling peanuts and peanut products having Florunner characteristics. Growers would be receptive to such a variety because they would know what to expect in matters such as yield, disease and pest resistance, and harvesting factors such as the plant growth habit, pod size and placement on the plant, and the incidence of hull splitting. Hulling and crushing could also be carried out with little or no adjustment to machine set-up and process parameters (cracking force, blanching conditions, incidence of split peanuts), as well as with a reasonable expectation of yield. Wholesalers, retailers, and end users would be highly receptive to peanut products whose only substantial distinguishing characteristic in comparison to Florunner was a dramatically improved fatty acid content